Mask ROM and process for fabricating the same

ABSTRACT

A Mask ROM is shown, including first resistors as a first part of memory cells, second resistors as a second part of memory cells, and contact plugs. Each first resistor includes: an undoped first poly-Si layer including an upper horizontal bar, a lower horizontal bar contiguous with the upper horizontal bar, and a step structure with a step height at which the two bars are contiguous, a spacer on the sidewall of the step structure, and a first silicide layer on the first poly-Si layer and being divided apart by the spacer. Each second resistor includes an undoped second poly-Si layer, and a contiguous second silicide layer on the second poly-Si layer. The contact plugs are disposed on the first silicide layer on the upper horizontal bar of each first poly-Si layer, and on the second silicide layer.

BACKGROUND OF THE INVENTION Field of Invention

This invention relates to a semiconductor apparatus, and particularly relates to a structure of a Mask ROM (read-only memory) and a process for fabricating the same.

Description of Related Art

A conventional Mask ROM includes MOS transistors as memory cells, where bit-line contact plugs are formed on source/drain (S/D) regions of the MOS transistors. However, current leakage easily occurs if a contact plug is misaligned, and the memory cell constituted of a MOS transistor occupies a large lateral area.

SUMMARY OF THE INVENTION

Accordingly, this invention provides a Mask ROM in which resistors instead of MOS transistors serve as memory cells.

This invention also provides a process for fabricating the Mask ROM.

The Mask ROM of this invention is shown, including first resistors as a first part of memory cells, second resistors as a second part of memory cells, and contact plugs. Each first resistor includes: an undoped first poly-Si layer including an upper horizontal bar and a step structure with a step height adjacent to the upper horizontal bar, a spacer on the sidewall of the step structure, and a first silicide layer on the first poly-Si layer and being divided apart by the spacer. Each second resistor includes an undoped second poly-Si layer, and a contiguous second silicide layer on the second poly-Si layer. The contact plugs are disposed on the first silicide layer on the upper horizontal bar of each first poly-Si layer, and on the second silicide layer.

In an embodiment of the Mask ROM of this invention, each first resistor further includes a material layer that is under the upper horizontal bar of the first poly-Si layer and causes the step height. The material layer may include an insulating layer.

The process for fabricating the Mask ROM of this invention includes the steps below. A plurality of undoped first poly-Si layers and a plurality of undoped second poly-Si layers are formed, wherein each first poly-Si layer includes an upper horizontal bar and a step structure with a step height adjacent to the upper horizontal bar. A spacer is formed on the sidewall of the step structure of each first poly-Si layer. A salicide (self-aligned silicide) process is performed to form a first silicide layer on each first poly-Si layer and a second silicide layer on each second poly-Si layer, wherein the first silicide layer is divided apart by the spacer to be non-contiguous on the first poly-Si layer, and the second silicide layer is contiguous on the second poly-Si layer. A plurality of contact plugs are formed on the first silicide layers on the upper horizontal bars of the first poly-Si layers, and on the second silicide layers.

In an embodiment, the process of this invention further includes: forming, before the first and the second poly-Si layers are formed, a patterned material layer for causing the step height. After the first and the second poly-Si layers are formed, the upper horizontal bar of each first poly-Si layers is located on the patterned material layer.

Since resistors are utilized as memory cells and the bit-line contact plugs are formed on the resistors in the Mask ROM of this invention, current leakage can be prevented even if a contact plug is misaligned. Moreover, the lateral area occupied by a resistor is remarkably smaller than the lateral area occupied by a MOS transistor in a conventional Mask ROM.

In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, a preferred embodiment accompanied with figures is described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 5 illustrate, in a top view as well as a cross-sectional view, a process for fabricating a Mask ROM according to an embodiment of this invention, wherein FIG. 5 also illustrates the structure of the Mask ROM according to the embodiment.

FIG. 2-1 illustrates an alternative for the structure of the substrate of the Mask ROM according to an embodiment of this invention.

DESCRIPTION OF EMBODIMENTS

This invention will be further explained with the following embodiments and the accompanying drawings, which are however not intended to restrict the scope of this invention. For example, though in the embodiments the poly-Si part of the word lines are formed simultaneously with the poly-Si layers of the resistor memory cells and the silicide part of the word lines are faulted simultaneously with the silicide layers of the resistor memory cells, in other embodiments, word lines may alternatively be formed over the silicide layers of the resistor memory cells after the silicide layers are formed.

FIGS. 1 to 5 illustrate, in a top view as well as a cross-sectional view, a process for fabricating a Mask ROM according to an embodiment of this invention, wherein FIG. 5 also illustrates the structure of the Mask ROM according to the embodiment.

Referring to FIG. 1, a patterned material layer 102 is formed over a substrate 100, in order to cause the step structure of each first poly-Si layer in the subsequent process. The substrate 100 may include a field oxide (FOX) layer that is formed on a semiconductor substrate, or include a semiconductor substrate. The material layer 102 may include an insulating layer, for example. The insulating layer may include, for example, a pad oxide layer and an SiN layer on the pad oxide layer.

Referring to FIG. 2, a plurality of undoped first poly-Si layers 104 a, a plurality of undoped second poly-Si layers 104 b, and a plurality of undoped third poly-Si layers 104 c for forming word lines later are formed over the substrate 100 and the material layer 102. Each first poly-Si layer 104 a includes a lower horizontal bar 104 a-1 that is connected with a third poly-Si layer 104 c that is for forming a word line later, an upper horizontal bar 104 a-2 on the material layer 102, and a step structure 1041 with a step height “h”, due to presence of the material layer 102. The lower horizontal bar 104 a-1 is contiguous with the upper horizontal bar 104 a-2 at the step structure 1041, which belongs to a portion of both the bars 104 a-2 and 104 a-1 and is located at the contiguous region in the middle part of the bar 104 a-2 and the bar 104 a-1. The top surface of the lower horizontal bar 104 a-1 is lower than that of the upper horizontal bar 104 a-2 by the step height h and at the same height as the bottom surface of the upper bar 104 a-2. Each second poly-Si layer 104 b includes a first portion 104 b-1 that is connected with a third poly-Si layer 104 c that is for forming a word line later, and a second portion 104 b-2.

The first poly-Si layers 104 a, the second poly-Si layers 104 b and the third poly-Si layers 104 c are undoped and do not require doping because a silicide layer will be formed on each of them later for electrical connection. The step height h may range from 300 Å to 500 Å. In addition, the first poly-Si layers 104 a, the second poly-Si layers 104 b and the third poly-Si layers 104 c may possibly be formed simultaneously with the poly-Si gate electrodes of logic devices (not shown).

When the substrate 100 includes include a semiconductor substrate, the first poly-Si layers 104 a, the second poly-Si layers 104 b and the third poly-Si layers 104 c are preferably formed after an insulating layer 106 is formed over the substrate 100, as shown in FIG. 2-1. The insulating layer 106 may be formed simultaneously with the gate dielectric of logic devices (not shown).

Referring to FIG. 3, a spacer 108 a is formed on the sidewall of the step structure 1041 of each first poly-Si layer 104 a, while spacers 108 b are formed on the other sidewalls that are present on the substrate 100. The spacer 108 a is directly contacting on a sidewall of the upper horizontal bar 104 a-2 and a portion of the top surface of the lower horizontal bar 104 a-1. One of the spacers 108 b is the spacer on the sidewall of the lower horizontal bar 104 a-1.

Referring to FIGS. 3 & 4, a salicide (self-aligned silicide) process is performed to form a first silicide layer 110 a on each first poly-Si layer 104 a, a second silicide layer 110 b on each second poly-Si layer 104 b, and a third silicide layer 110 c on each third poly-Si layer 104 c. The first silicide layer 110 a is divided apart by the spacer 108 a to be non-contiguous on the first poly-Si layer 104 a. The second silicide layer 110 b is contiguous on the second poly-Si layer 104 b.

Each third silicide layer 110 c and the underlying third poly-Si layer 104 c serve as a word line. Each first silicide layer 110 a includes a first portion 110 a-1 that is on the lower horizontal bar 104 a-1 of a first poly-Si layer 104 a and is connected with a word line 110 c, and a second portion 110 a-2 that is on the upper horizontal bar 104 a-2 of the first poly-Si layer 104 a and is disconnected from the first portion 110 a-1 due to presence of the spacer 108 a. Each second silicide layer 110 b includes a first portion 110 b-1 that is connected with a word line 110 c, and a second portion 110 b-2 that is connected with the first portion 110 b-1. Thereby, a plurality of first resistors 12 each based on a non-contiguous silicide layer 110 a, and a plurality of second resistors 14 each based on a contiguous silicide layer 110 b are made, serving as a first part of memory cells that represent “1” (or “0”) and a second part of memory cells that represent “0” (or “1”), respectively.

Referring to FIG. 5, a plurality of contact plugs 112 are formed on the second portions 110 a-2 of the first silicide layers 110 a on the upper horizontal bars 104 a-2 of the first poly-Si layers 104 a, and on the second portions 110 b-2 of the second silicide layers 110 b. A plurality of bit lines 114 are formed, each being connected with the contact plugs 112 of a column of resistors 12/14. The height or thickness h1 of each upper horizontal bars 104 a-2 is slightly smaller than the step height h between the top surfaces of the upper and lower bars 104 a-2 and 104 a-1 by the thickness of the first silicide layer 110 a.

For a first resistor 12 that is based on a non-contiguous silicide layer 110 a, because the corresponding word line 110 c and the corresponding bit line 114 are connected with disconnected portions 110 a-1 and 110 a-2 of the non-contiguous silicide layer 110 a, respectively, a current cannot flow between the corresponding word line 110 c and the corresponding bit line 114. On the contrary, for a second resistor 14 that is based on a contiguous silicide layer 110 b, because the corresponding word line 110 c and the corresponding bit line 114 are connected with connected portions 110 b-1 and 110 b-2 of the contiguous silicide layer 110 b, respectively, a current can flow between the corresponding word line 110 c and the corresponding bit line 114. Thus, it is possible that each first resistor 12 represents “1” and each second resistor 14 represents “0”, or each first resistor 12 represents “0” and each second resistor 14 represents “1”.

Since resistors are utilized as memory cells and the bit-line contact plugs are formed on the resistors in the Mask ROM of this invention, current leakage can be prevented even if a contact plug is misaligned.

Moreover, the lateral area occupied by a resistor is remarkably smaller than the lateral area occupied by a MOS transistor in a conventional Mask ROM.

This invention has been disclosed above in the embodiments, but is not limited to those. It is known to people of ordinary skill in the art that some modifications and innovations may be made without departing from the spirit and scope of this invention. Hence, the scope of this invention should be defined by the following claims. 

What is claimed is:
 1. A Mask ROM, comprising: a plurality of first resistors as a first part of memory cells, each of which includes: an undoped first poly-Si layer including: an upper horizontal bar, a step structure with a step height, and a lower horizontal bar that is contiguous with the upper horizontal bar at the step structure and has a top surface being lower than a top surface of the upper horizontal bar by the step height and at same height as a bottom surface of the upper horizontal bar, wherein the step structure belongs to a portion of both the upper horizontal bar and the lower horizontal bar, a spacer directly contacting on a sidewall of the upper horizontal bar and a portion of the top surface of the lower horizontal bar, and a first silicide layer on the undoped first poly-Si layer, wherein the first silicide layer is divided apart by the spacer to be non-contiguous, and a height of the upper horizontal bar is smaller than the step height by a thickness of the first silicide layer; a plurality of second resistors as a second part of the memory cells, each of which includes an undoped second poly-Si layer and a contiguous second silicide layer on the undoped second poly-Si layer; and a plurality of contact plugs, disposed on the first silicide layer on the upper horizontal bar of each undoped first poly-Si layer, and on the second silicide layer.
 2. The Mask ROM of claim 1, wherein each first resistor further includes a material layer that is under the upper horizontal bar of the undoped first poly-Si layer and causes the step height.
 3. The Mask ROM of claim 2, wherein the material layer comprises an insulating layer.
 4. The Mask ROM of claim 3, wherein the insulating layer comprises a pad oxide layer and an SiN layer on the pad oxide layer.
 5. The Mask ROM of claim 1, wherein first portions of the second silicide layer and the first silicide layer on the lower horizontal bars of the undoped first poly-Si layers are connected with a plurality of word lines, and second portions of the second silicide layer and the first silicide layer on the upper horizontal bars of the undoped first poly-Si layers are connected with a plurality of bit lines.
 6. The Mask ROM of claim 1, wherein the first resistors and the second resistors are disposed on a field oxide (FOX) layer.
 7. The Mask ROM of claim 1, wherein the first resistors and the second resistors are disposed on a semiconductor substrate and separated from the semiconductor substrate by an insulating layer.
 8. The Mask ROM of claim 7, wherein the insulating layer is formed simultaneously with gate dielectric of logic devices.
 9. The Mask ROM of claim 1, wherein the undoped first poly-Si layers and the undoped second poly-Si layers are formed simultaneously with poly-Si gate electrodes of logic devices.
 10. The Mask ROM of claim 1, wherein the step height ranges from 300 Å to 500 Å.
 11. A Mask ROM, comprising: a plurality of first resistors as a first part of memory cells, each of which includes: an undoped first poly-Si layer including: an upper horizontal bar, a step structure with a step height, and a lower horizontal bar that is contiguous with the upper horizontal bar at the step structure and has a top surface being lower than a top surface of the upper horizontal bar by the step height and at same height as a bottom surface of the upper horizontal bar, wherein the step structure belongs to a portion of both the upper horizontal bar and the lower horizontal bar, a first spacer directly contacting on a sidewall of the upper horizontal bar and a portion of the top surface of the lower horizontal bar, and a second spacer on a sidewall of the lower horizontal bar, and a first silicide layer on the upper horizontal bar and the lower horizontal bar of the undoped first poly-Si layer, wherein the first silicide layer is divided apart by the first spacer to be non-contiguous, and a height of the upper horizontal bar is smaller than the step height by a thickness of the first silicide layer; a plurality of second resistors as a second part of the memory cells, each of which includes an undoped second poly-Si layer and a contiguous second silicide layer on the undoped second poly-Si layer; and a plurality of contact plugs, disposed on the first silicide layer on the upper horizontal bar of each undoped first poly-Si layer, and on the second silicide layer. 